A synchronous machine is a rotating AC machine, with the excitation field produced by permanent magnets in the rotor. This machine operates as a motor when its stator winding is fed with three-phase current. The rotor synchronously rotates with the rotating field, irrespective of the magnitude of the load torque. When used as a synchronous generator, this machine must be driven by a shaft, so as to convert the mechanical energy to electrical energy which is fed from the stator into the mains system. Operating in this manner has a disadvantageous affect on the torque characteristic, on the loss balance and on the noise of the synchronous machine. These drawbacks are produced, inter alia, by harmonic fields from a relatively large number of poles, which are superimposed on the fundamental field.
For three-phase synchronous machines, windings are known which have different coil widths in order to avoid harmonic fields being induced in the windings. These are interleaved windings, located in slots N1. The number of slots N1 is in this case greater than three times the number of poles 2p. N1>3·2p.
There are also windings with low-cost tooth coils. In this case, one coil is fitted around one stator tooth. The advantage over interleaved windings is, first, the simplified winding technology and, second, the avoidance of contact between windings in the end winding area. However, one disadvantage is the poor winding factor for harmonic fields. In the case of a tooth coil winding having a number of holes q=½, that is to say N1=1.5·2p, the winding factors (wifa) for the fifth and seventh harmonic are very high, wifa (5th/7th)=0.806. This leads to increased cogging in corresponding motors.